In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or imaging surface. The imaging surface can be the surface of a rotating member, such as a rotating drum or belt, or it can be a layer of material mounted to a rotating drum or belt, which is called a “blanket” in this document. “Imaging surface” refers to both the surface of a rotating member and a blanket in this document.
A print cycle in these indirect printers typically includes preparation of the imaging surface for printing, formation of the ink image on the treated imaging surface, preparation of the ink image for transfer to media, transfer of the ink image from the imaging surface to the media, and treatment of the image on the media before egress of the media from the printer. The cycle is repeated for subsequent images. Operation of components that perform one or more portions of the print cycle can be monitored and adjusted with reference to image data generated by directing light towards the imaging surface and detecting the amplitude of the reflected light with an optical sensor. These optical sensor image data can be processed to distinguish ink from imaging surface background to identify position and size of the ink on the imaging surface. From these measurements, a controller can determine whether the surface treatment components, printheads, and transfer components are working within an acceptable range and, if necessary, adjust the components to bring their operation within an acceptable range.
The imaging surfaces in inkjet printers may have structure that causes significant variations in the reflected light. When these reflectivity variations are close to the variations caused by the contrast between the colorant and the substrate, the precision of the image quality measurement is adversely affected. Changes in the imaging surface velocity, which are not uncommon, also make alignment of the two images difficult. Methods and systems that are more robust with regard to imaging surface velocity variations would be beneficial.